Illegal! I am aware that handicraft works projects just do and wait for legal issues to come to them, but if you do commercial products, you have to go the other way around. Fines are expensive in aviation, so I prefer to stay clear of these approaches.
As a remark, the increasing frequency congestion, which already lead to UAT in the US, is now picking up in Europe as well and the more power you put in each transmitter, the worse it gets. Yes, free bands are one possibility, but there is none internationally free for all.
1090ES is not the problem, as it is one of the few internationally agreed frequency. The other frequencies and various legal restrictions are. I.e. the Flarm community had to develop quite some (expensive in development) mechanisms to get the widespread coverage it has now in Europe. PowerFlarm does monitor the GPS position, automatically switches the Flarm part off when crossing border and leaving ADS-B In on. Flarm is neither allowed in the US and Canada due to the frequency used and nor in US registered aircraft or aircraft carrying people living in the US or Canada or being US or Canadian citizens (see the relevant paragraphs in the PowerFlarm manuals for reference). I assume the same holds for P3I devices on the same frequency with similar surveillance function, but I have not seen similar documentation in the Pilotaware.